High vacuum casting with electron bombardment heating



P 1966 c. w. HANKS ETAL 324-7 554 HIGH VACUUM CASTING WITH ELECTRONBOMBARDMENT HEATING Filed Aug. 6, 1962 Paws? JuPPLY SUPPLY INVENTORfi(MW; 55 w, firm/M Alva/1 miM/mg 1e;

3,247,554 HIGH VACUUM CASTING WITH ELECTRON BOMBARDMENT HEATING CharlesW. Hanks, Orinda, and Hugh R. Smith, In, Oakland, Ca'lif., assignors toStaulier Chemical Company, New York, N.Y., a corporation of DelawareFiled Aug. 6, 1962, Ser. No. 219,353 1 Claim. (Cl. 22-572) The presentapplication is a continuation-in-part of our copending applicationSerial No. 764,357, filed September 30, 1958, now abandoned, which is acontinuation of our then copending application Serial No. 600,561, whichissued December 6, 1960 as US. Patent No. 2,963,530.

This invention relates to the casting of metals and the like in a highvacuum, and in particular to improved apparatus for the continuouscasting of sound, void-free rod-like ingots of highly devolatilizedmaterial.

Metals and the like that have been cast in a high vacuum, generally atan absolute pressure of one micron of mercury or less, have exceptionalcharacteristics because of the high degree of purity anddevolatilization which can thus be obtained. The object of thisinvention is to provide improved apparatus for casting metals and thelike in a high vacuum, and in particular to apparatus capable ofcontinuously casting sound, void-free ingots of highly devolatilizedmaterial.

Briefly stated, this object is achieved by continuously supplying moltenmaterial into the open top of a liquidcooled annular mold, situated in ahigh vacuum tank, from the bottom of which the cast material can becontinually withdrawn as a rod-like ingot. In order to produce a sound,void-free ingot, it is necessary to maintain a pool of molten materialin the top of the mold, so that the molten material supplied into themold will not solidify too quickly, but rather solidification andcrystallization of the material takes place continuously at the bottomof the pool. Maintaining such a pool presents a difiicult heatingproblem in the high vacua which must be provided for the desired degreeof devolatilization. Electric arc heating requires too high a pressure,and induction heating and the like is prevented by the short circuitingaction of the casting mold. According to the present invention themolten pool is maintained by electron bombardment heating from anannular cathode disposed in vertical alinement above the open top of thecasting mold, and the molten material is continually supplied into thecasting mold by dropping it through the annular cathode structure.

' The invention will be better understood from the following detaileddescription of an illustrative embodiment taken in connection with theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of an installation for the continuousproduction of homogeneous ingots from material initially supplied insponge form,such as titanium sponge; and

FIG. 2 is a transverse section taken along the line 2--2 through thecathode structure disposed above the casting mold.

In the diagram of FIG. I, illustrative of a complete installationincluding associated equipment not directly a part of the presentinvention, the various elements are shown in purely schematic form,without indication of their actual structure. The elements illustratedare either conventional, so that their structure is well known, or willbe described in'detail hereinafter.

In FIG. 1 the vacuum system comprises a main treatment chamber or tank1, and a vacuum lock 3, these being provided with exhaust portscommunicating with ducts 5 and 7 respectively, leading to exhaust pumpsred Sates Patent lock 3 through a wide feed passage 13 that may beclosed and sealed by a gate valve 15. The vacuum lock chamher 3preferably has a relatively small volume as compared with the maintank 1. It is provided with a loading gate 17, also preferably avacuum-tight gate valve, through which the material to be treated can beintroduced.

Immediately below the gate 17 is positioned a feed hopper 19 thatsupplies a vibratory feed mechanism 21. These parts are so arranged thatthe apparatus 21 operates to feed divided material 22 from the hopperolf of its end and drop it through the channel 13 when the gate valve 15is open. Material falling through the gate valve is received by a secondhopper 23 within the main tank. Hopper 23 also is shown supplying avibratory feed mechanism 25, adapted to supply a flow of material to theapparatus with which the present invention is directly concerned at thedesired rate.

As stated, this apparatus is conventional and its oper ationis wellunderstood. To place a charge of material for treatment in it, gatevalve 15 is closed, valve 17 is opened and hopper 19 is filled with acharge of the material to be treated. Valve 17 is then closed and thelock is exhausted through duct 7 by pump 11 to substantially the samedegree of vacuum as that at which the main tank 1 is maintained. Thisevacuation removes the major portion of the gas held in the intersticesof the sponge and the interspaces between particles. Pump ill hassufiicient capacity to accomplish this evacuation be fore hopper 23 canbe emptied at the prescribed rate of feed. When the required vacuum hasbeen established within the vacuum lock 3, valve 15 is open and the feed'21 is activated. The operation of the feed is adjusted so that itoperates much more rapidly than the feed 25 within the main tank,filling the hopper 23 before the charge in hopper 19 is exhausted. Theoperation is then repeated, feed 25 working continuously while feed 19works intermittently and the vacuum in the main tank 1 is not broken atall during the operation of the apparatus.

The pump 9 should have sufficient capacity to reduce the base pressurein the tank 1 to a fraction of 21 micron, and to keep the operatingpressure within the tank, when gas is being evolved, to less than onemicron Hg. It follows that pump 11 should be able to reduce the pressurewithin the vacuum lock to the operating pressure within the timeallowance given by the capacity of the hopper 23.

The continuous feed from the vibrator mechanism 2.5

supplies a third small hopper 27, the bottom of which opens directlyinto a duct 29 containing the feed screw 31 of a screw conveyor. Theshaft 33 of the conveyor extends outwardly through the rear end of duct29 and through a vacuum-tight seal or gland 35, where it connects to adrive motor 37. The latter is preferably of the adjustable-speed type,although this may not be necessary in an installation adapted to handlematerial of a constant grade and average gas content.

The feed screw 31 forces the material into the forward end 39 of theduct. This end at least of the duct must be of conducting material; theremainder of the duct may be, and usually will be, of metal as well. Itmust also be able to withstand the temperature of the molten metal atits outlet or discharge end and to withstand possible chemical attack ordissolution in the molten metal. some punposes graphite will meet theserequirements. Preferably, however, this discharge end of the duct ismade of metal of high heat conductivity, usually copper, and isfluid-cooled as, for example, by a water-jacket 4'1 connecting throughleads 43 to a water circulating system. The cooling system must befluid-tight; in one embodiment of the invention the jack and ducts areHeliarc" Welded of copper to insure continuity.

satia i- Patented-Apr. 26, 1966 I For i The duct is shown in the presentcase as being horizontal. It is generally preferable that it extendgenerally .transversely across the tank 1, but it may slant upward ordownward. It is also generally preferable that the opening in thedischarge end should face generally upward and in nearly all cases itshould be inclined with respect to the horizontal as shown. Thepreferred angle of the opening for most purposes lies between 30 and 80degrees with the horizontal. With a tubular duct the slanted orificeprovides a pouring lip which guides the molten metal from the fusionzone to drop in a definite course instead of falling in a more or lessrandom fashion from any point of the melted surface. It should bementioned, however, that because of the high surface tension of themelted material it has proved possible to maintain a complete liquidseal covering an overhanging surface, and that it has also provedpossible to maintain a seal and practice the invention using a verticalduct fed from below to form a horizontal liquid layer. The use of atransverse feed and upward-facing discharge are therefore precautionarymeasures that lead to simpler mechanical structures, rather thannecessary features of the invention.

A thermionically-emissive cathode structure, indicated generally in FIG.1 by the reference character 44, is mounted with its emissive surfacespaced from and generally parallel to the discharge orifice of the duct.Various forms of structure have been used for the cathode, and havegiven successful operation.

An ingot mold 54 is positioned within the tank, immediately below thelip formed at the discharge end of the duct 39. The mold can be formedin the same general manner as the end 39 of the duct, i.e., of copperwith a surrounding water-jacket supplied by tubing 54' connecting with awater supply. The mold is open at the bottom and immediately below it isa seal 55 through which an ingot 57, formed within the mold, can becontinuously withdrawn without breaking the vacuum in the tank 1. Athermo-emissive cathode 59 is mounted immediately above the opening inthe top of the ingot mold. The cathode can be formed of a single turn oftungsten wire or tape, substantially coaxial with the mold 54, andsurrounded by an annular focusing shield 62, having an opening throughwhich the material can fall to the top of the ingot.

In operation the conveyor screw 31 feeds the divided materialcontinuously to the forward or right end 39 of the transverse feed duct.Here the material is melted by an electric discharge from the cathodestructure 44, and as the material melts it drops, as shown, through theannular cathode 59 and into the open top of casting mold 54. Thus, assubstantially continuous supply of molten material is fed into the topof the casting mold.

' The bottom of the ingot mold 54 is plugged by the ingot 57, so thatthe mold forms a cup into which the metal drops. In starting theapparatus for the first time it can be plugged by any other material ofproper dimension, e.g., a steel shaft which extends through seal 55 andinto the mold, later to be discarded. Where the melted material touchesthe water-chilled wall of the mold it immediately solidifies, as was thecase with the water-cooled duct, and therefore does not attack the mold.Enough discharge is maintained from the cathode 59 to the forming ingotto maintain a small pool of melted material at the upper end of themold, into which the discharge from the duct falls and is promptlydiffused therein, even though it may have cooled slightly, by radiation,in its fall. -It is generally desirable that the molten pool just coverthe top of the ingot 57, so that there is minimum contact between themolten material and the walls of mold 54, and yet the entire uppersurface of the ingot is covered with molten material, which is in thecontinuous iii process of solidifying at its boundary with the ingot,whereby sound, void-free castings are formed. Only enough power issupplied to the discharge at the mouth of the mold to maintain a smallliquid pool of this character at that point. This pool gradually freezesas the ingot is withdrawn, leaving no definite lines of cleavage orlamine but instead of a completely homogeneous ingot which can berolled, drawn or subjected to any other treatment required for furtherfabrication.

A preferred form of cathode is a horizontal loop 59 of tungsten wire,which is connected through leads 60 and 61 to the electrical powersupply and is heated to incandescence by current through the loopprovided by the power supply. In consequence of its high temperature thetungsten loop 59 thermionically emits a copious supply of electrons.Furthermore, cathode 59 is maintained at a substantial negativepotential, typically 15,000 or 20,000 volts, by the power supply 53,whereas the molten material within mold 54 is maintained at groundpotential by electrical contact with the mold and the cast ingot 57,both of which are electrically connected to ground. Hence, the electronsare accelerated to high velocities by the applied voltage, and bombardand thereby heat the upper surface of the molten pool as an anode. Forfocusing the electrons on to the molten pool and preventing undesirablebombardment of the vacuum tank walls and other parts, a generallyannular metal shield 62 surrounds the cathode in the manner shown. Aconnection 63 between this shield and the center of loop 59 maintainsthe metal shield substantially at cathode potential, and thereby theelectrons thermionically emitted by loop 59 are focused on to the uppersurface of the molten pool atop ingot 57 within casting mold 54.

The invention is not limited to the specific example herein illustratedand described, since it is contemplated that various changes andmodifications will be apparent to those skilled in the art. The scope ofthe invention is defined by the following claim.

We claim:

Vacuum melting and casting apparatus comprising a vacuum tank, means forevacuating the tank, a casting mold with an open topdisposed in thetank, a horizontal annular cathode disposed within the tank in closelyspaced relationship above and in vertical alignment with the open top ofthe mold, an annular metal shield open at the top and bottom thereof andsurrounding said cathode, electrical connections for maintaining saidshield substantially at cathode potential, means for supporting materialto be melted over the mold and above the annular cathode, means forheating the material to melt it and cause it to drop in molten formthrough the annular cathode and into the open top of the mold, means forheating the cathode to a temperature to cause it to emit electrons, andmeans for accelerating electrons from the cathode and directing theminto the top of the mold so they strike material in the upper portion ofthe mold to maintain a pool of molten material which receives moltenmaterial dropping into the mold.

References Cited bythc Examiner UNITED STATES PATENTS 2,554,902 5/1951Godley 3 l5106 2,858,586 11/1958 Brennan 22-57.Z 2,880,483 4/1959 Hankset al. 22-572 2,935,395 5/1960 Smith -10 2,963,530 12/ 1960 Hanks et al13-31 J. SPENCER OVERHOLSER, Primary Examiner.

MICHAEL V. BRINDISI, MARCUS U. LYONS,

Examiners.

